FIG. 13 shows a first conventional loudness compensator disclosed in Japanese Patent Application (OPI) No. 248611/86 (the term "OPI" as used herein means an "unexamined published application"). The compensator comprises a sound-level-controlling tapped variable resistor VR.sub.1 and a CR filter circuit 1. A high-frequency sound range compensation capacitor C.sub.1 is connected between the intermediate tap c and signal input terminal of the variable resistor VR.sub.1. A low-frequency sound range compensation capacitor C.sub.2 and a resistor R.sub.1 are connected in series with each other between the intermediate tap c and other terminal b of the variable resistor VR.sub.1 and connected in parallel with a low-frequency sound range compensation level-limiting resistor R.sub.2.
FIG. 14 shows a second conventional loudness compensator disclosed in Japanese Patent Application (OPI) No. 223909/83. The compensator has a sound-levelcontrolling tapped variable resistor VR.sub.1. A resonance circuit 2 is connected between the intermediate tap c and terminal b of the variable resistor VR.sub.1.
FIG. 15 shows a third conventional loudness compensator disclosed in Japanese Patent Application (OPI) No. 223909/83. The compensator has a pair of sound-level-controlling variable resistors VR.sub.1 and VR.sub.1 ' for two channels, and a third controlling variable resistor VR.sub.2. which is operated in conjunction with the preceding variable resistors. The level of the output from a resonance circuit 2 is regulated by the third controlling variable resistor VR.sub.2. The compensator also has addition circuits 3 and 3' differentially connected to the outputs of the sound-level-controlling variable resistors VR.sub.1 and VR.sub.1.
FIG. 16 shows a fourth conventional loudness compensator disclosed in Japanese Utility Model Application (OPI) No. 95723/85. The compensator comprises a plurality of filters 4 and 5 for dividing an audio input signal into two or more frequency range components, a logarithmic compression amplifier 6 which logarithmically compresses at least one of the outputs from the filters, and an addition circuit 8 which adds the output from the amplifier 6 to the other filter output processed by a flat amplifier 7.
When the slider d of the sound-level-controlling variable resistor VR.sub.1 of the first conventional loudness compensator is slid toward the intermediate tap c thereof between the signal input terminal a and the tap c, the sound frequency-pressure characteristic is such that the level of sound pressure is heightened by an equal quantity in both a low and a high range of sound frequency. When the slider d is slid between the signal input terminal a and the tap c in such a manner that the resistance between the tap and the slider is gradually increased, the resistance between the signal input terminal and the slider is gradually decreased so that the diagrammatic curve of the sound frequency-pressure characteristic is made gradually flatter as shown in FIG. 17. Although the second conventional loudness compensator is similar to the first one in these respects, the diagrammatic curve of the sound frequency-pressure characteristic has a larger slope due to the resonance circuit 2 and is shorter in the entire range of sound frequency, as shown in FIG. 18. When the sliders d and d' of the sound level controlling variable resistors VR.sub.1 and VR.sub.1 ' of the third conventional loudness compensator are slid toward the signal input terminals a and a' of the resistors so that the sound levels are heightened, the slider g of the second controlling variable resistor VR.sub.2 which is operated in conjunction with the preceding variable resistors is slid to the terminal f of the resistor VR.sub.2 so that the level of the output from the resonance circuit 2 is gradually lowered as shown in FIG. 19. In the fourth conventional loudness compensator, the frequency range component output from the filter 4 is linear to the audio input signal, and the frequency range component output from the other filter 5 is logarithmically compressed and then added to the output from the preceding filter, so that the diagrammatic curve of the sound frequency-pressure characteristic is made gradually flatter as shown in FIG. 20.
The drawback of the first and the second conventional loudness compensators including the sound level controlling tapped variable resistors is that the sound frequency-pressure characteristic is such that the level of sound pressure is heightened by an equal quantity in the lower and the higher ranges of sound frequency relative to that of the sound pressure in the middle range of sound frequency as shown in FIG. 17, when the sound level is not higher than a value corresponding to the position of the tap of the variable resistor (that position is usually at an angle of 120.degree. in the case where the maximum rotative angle of the variable resistor is 300.degree.). Particularly in the first conventional loudness compensator having the CR filter circuit 1, the slope of the diagrammatic curve of the sound frequency-pressure characteristic is 6 dB/oct and there is a pressure level fluctuation in the frequency range of 200 Hz or more so that the compensator has the particular disadvantage that the sound compensated through the compensator is unclear or the middle-frequency sound through the compensator is weak.
As for the third conventional loudness compensator having the second controlling variable resistor VR.sub.2. the quantity of the increase in the loudness in the low-frequency sound and/or the high-frequency sound can be gradually changed for adjustment by the second controlling variable resistor. For that reason, the compensator does not have the above-mentioned drawback and disadvantage. However, a drawback is that the compensator needs three or four mutually-coupled variable resistors, so that the spatial efficiency thereof is lower and the rotation thereof does not feel good.
As for the fourth conventional loudness compensator, the diagrammatic curve of the sound frequency-pressure characteristic is similar to an equal-sensation curve (which is a Fletcher-Manson equal-loudness curve) at a low sound level. For that reason, a drawback is that the quantity of compensation by the compensator is excessive for listening to the reproduced sound from a music source on the market or from the like, and the quality of the sound is therefore unnatural. In the process of production of the music source on the market, sounds are usually subjected to multiple-track recording through multiple microphones, quality adjusted by an effect producing device or the like. and mixed-down into a two-channel stereo format. At that time, a person listens to the recorded sounds at a certain level through a monitoring reproduction device in a studio. The sound level is usually as high as about 100 phons. The customer purchases the music source to listen to the sounds through his sound reproduction device. If the customer listens to the sounds at nearly the same sound level as the other person listens to them through the monitoring reproduction device in the studio, the customer obtains nearly the same acoustic quality as the other person does, under ideal conditions (in reality, the acoustic quality depends on the sound level characteristics of the ears, the acoustic characteristics of the room and the properties of the sound reproduction device). Although the loudness of the reproduced sounds is usually much lower than 100 phons, the sound level is higher than 100 phons in some cases and is, for example, 120 phons.
FIG. 3 shows Robinson-Datson curves which are equal-sensation curves at the sound levels of pure sounds. It is understood from FIG. 3 that the form of the Robinson-Datson curve at the sound level of 100 phons is different from that of the curve at the sound level of 120 phons. In other words, the rise in the level of sound pressure in a low range of frequency or the fall in the sensitivity of the ear to the level of sound pressure in the range is relatively steep at the sound level of 100 phons but relatively low at the sound level of 120 phons. This means that the quality of a sound reproduced at a high sound level is not equal to that of the sound monitored by a person at the time of recording of the sound. In other words, the level of the pressure of the reproduced sound in a low range of sound frequency or a high range of sound frequency feels higher at the high sound level. Therefore, the quality of the reproduced sound at the high sound level is different from that of the reproduced sound at a low level.
FIG. 4 shows the differences between the equal-sensation curve at the reference sound level of 100 phons and the other equal-sensation curves at other levels. It is understood from FIG. 4 that the level of sound pressure rises or the sensitivity of the ear thereto falls in the low-frequency sound range of 200 Hz and less and the middle-frequency and high-frequency sound ranges from 1.5 kHz to 10 kHz as the sound level falls, and that the level of sound pressure falls or the sensitivity of the ear thereto rises in the sound ranges as the sound level rises. For that reason, the differences need to be compensated if the quality of the reproduced sound is to be nearly equal to that of the sound monitored by the person at the time of the recording of the sound.
Each of the conventional loudness compensators including the sound level controlling tapped variable resistors function so that the diagrammatic curve of the sound frequency-pressure characteristic is made flat when the variable resistor is set for the highest sound level. For that reason, a drawback of the compensators is that the compensation becomes ineffective when listening to the sounds of the music source on the market or to the like, at a high sound level which makes the quality of the sounds unnatural.
All of the four conventional loudness compensators have drawbacks in that they do not perform compensation at a sound level higher than that of the reference equal-sensation curve or compensate for masking based on the noise produced by a moving vehicle equipped with a sound reproduction device provided with the loudness compensator.